Cell Division - Meiosis

From Embryology

Introduction

Mitosis and meiosis
Meiosis Links: Spermatogenesis | Oogenesis | Mitosis

Some Recent Findings

Mitosis and Meiosis

Mitosis meiosis1.jpg Progeny

Mitosis 2 Daughter cells identical to parent (diploid)

Meiosis Germ cell division (haploid)

  • Reductive division
  • Generates haploid gametes (egg, sperm)
  • Each genetically distinct from parent
  • Genetic recombination (prophase 1)
    • Exchanges portions of chromosomes maternal/paternal homologous pairs
  • Independent assortment of paternal chromosomes (meiosis 1)

Cell Birth - Mitosis and Meiosis 1st cell division- Meiosis

Homologous chromosomes pairing unique to meiosis

  • Each chromosome duplicated and exists as attached sister chromatids before pairing occurs
  • Genetic Recombination shown by chromosomes part red and part black
    • chromosome pairing in meiosis involves crossing-over between homologous chromosomes

(For clarity only 1 pair of homologous chromosomes shown)

Comparison of Meiosis/Mitosis

  • After DNA replication 2 nuclear (and cell) divisions required to produce haploid gametes
  • Each diploid cell in meiosis produces 4 haploid cells (sperm) 1 haploid cell (egg)
  • Each diploid cell mitosis produces 2 diploid cells

Meiosis Sex Differences

Female (oogenesis)

  • Meiosis initiated once in a finite population of cells
  • 1 gamete produced / meiosis
  • Completion of meiosis delayed for months or years
  • Meiosis arrested at 1st meiotic prophase and reinitiated in a smaller population of cells
  • Differentiation of gamete occurs while diploid in first meiotic prophase
  • All chromosomes exhibit equivalent transcription and recombination during meiotic prophase

Male (spermatogenesis)

  • Meiosis initiated continuously in a mitotically dividing stem cell population
  • 4 gametes produced / meiosis
  • Meiosis completed in days or weeks
  • Meiosis and differentiation proceed continuously without cell cycle arrest
  • Differentiation of gamete occurs while haploid after meiosis ends

Sex chromosomes excluded from recombination and transcription during first meiotic prophase

Female Gametogenesis

In females, the total number of eggs ever to be produced are present in the newborn female.

  1. All eggs are arrested at an early stage of the first meiotic division as a primary oocyte (primordial follicle). Following purberty, during each menstrual cycle, pituitary gonadotrophin stimulates completion of meiosis 1 the day before ovulation.
  2. In meiosis 1, a diploid cell becomes 2 haploid (23 chromosomes) daughter cells, each chromosome has two chromatids. One cell becomes the secondary oocyte the other cell forms the first polar body.
  3. The secondary oocyte then commences meiosis 2 which arrests at metaphase and will not continue without fertilization.
  4. At fertilization meiosis 2 completes, forming a second polar body. Note that the first polar body may also undergo this process forming a third polar body.

Female gametogenesis

Female Abnormalities

Trisomy 21 female karyotype

Meiotic non-disjunction resulting in aneuploidy, most are embryonic lethal and not seen. The potential for genetic abnormalities increase with maternal age.

  • Autosomal chromosome aneuploidy
    • trisomy 21 - Down syndrome
    • trisomy 18 - Edwards syndrome
    • trisomy 13 - Patau syndrome
  • Sex chromosome aneuploidy
    • monosomy X - Turner's Syndrome
    • trisomy X - Triple-X syndrome
    • 47 XXY - Klinefelter's Syndrome

Male Gametogenesis

In males, sperm continues to be generated throughout life from a stem cell population in the testis. Spermatozoa maturation involves two processes meiosis and spermiogenesis Male gametogenesis.jpg

The above figure compares meiosis to the female (the polar bodies have been removed and labelling updated).

Human Spermatozoa Development

  • Spermatogenesis process of spermatagonia mature into spermatazoa (sperm).
  • Continuously throughout life occurs in the seminiferous tubules in the male gonad- testis (plural testes).
  • At puberty spermatagonia activate and proliferate (mitosis).
  • about 48 days from entering meiosis until morphologically mature spermatozoa
  • about 64 days to complete spermatogenesis, depending reproduction time of spermatogonia
  • follicle stimulating hormone (FSH) - stimulates the spermatogenic epithelium
  • luteinizing-hormone (LH) - stimulates testosterone production by Leydig cells

Human-spermatozoa EM01.jpg

Spermatozoa animation icon.jpg Mature human spermatozoa
  • 60 µm long, actively motile
  • divided into 3 main regions (head, neck and tail)
  • head - (flattened, 5 µm long by 3 µm wide) the nucleus and acrosome. Posterior part of nuclear membrane forms the basal plate.
  • neck - (1 µm) attached to basal plate, transverse oriented centriole, contains nine segmented columns of fibrous material, continue as outer dense fibres in tail.
  • tail - 3 parts a middle piece, principal piece and end piece
    • middle piece - (5 µm long) axonema and dense fibres surrounded by mitochondria
    • principal piece - (45 µm long) fibrous sheath interconnected by regularly spaced circumferential hoops
    • end piece - (5 µm long) axonema surrounded by small amount of cytoplasm and plasma membrane

Links: Spermatozoa Development | MBoC - Sperm | MBoC - Highly simplified drawing of a cross-section of a seminiferous tubule in a mammalian testis | MBoC - Cytoplasmic bridges in developing sperm cells and their precursors

Puberty

  • In humans at puberty, hormonal and morphological changes occur within the gonad and other systems (secondary sex characteristics).
  • Within the testis the immature Sertoli cells cease to proliferate and differentiate.
  • Spermatogonium proliferate and spermatogenesis begins.
  • It takes about 70 days for cells to mature from the diploid spermatogonium to a primary spermatocyte.
  • This maturation occurs in waves along the seminiferous tubules.

Ejeculate

Azoospermia - Non-obstructive azoospermia (NOA) and Obstructive azoospermia (OA)
  • release of spermatozoa and accessory gland secretions from the male genital tract (3.5 ml)
  • 200-600 million sperm, by volume less than 10 % spermatozoa
  • Accessory Gland secretions - 60 % seminal vesicle, 30 % prostate and 10 % bulbourethral

Male Abnormalities

  • Oligospermia - (Low Sperm Count) less than 20 million sperm after 72 hour abstinence from sex
  • Azoospermia - (Absent Sperm) blockage of duct network
  • Immotile Cilia Syndrome - lack of sperm motility


Abnormalities

Meiotic Nondisjunction

  • Occurs when homologues fail to separate during meiotic division I or II
  • Down Syndrome
  • Caused by an extra copy of chromosome 21

Nondisjunction

Chromosomal Translocations

  • Philadelphia chromosome
  • Chronic myelogenous leukemia
    • Piece of Chr9 exchanged with Chr22 Generates truncated abl

Overstimulates cell production

References

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Cite this page: Hill, M.A. (2024, March 28) Embryology Cell Division - Meiosis. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Cell_Division_-_Meiosis

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G